Implications of measures of reliability for theories of priming: Activity in semantic memory is inherently noisy and uncoordinated

In experiments on semantic priming, participants vary substantially in the absolute magnitude of priming they produce. Are these individual differences systematic or do they arise from random processes, and does the answer carry theoretical implications? To find out, we examined split‐half and test–retest reliability of semantic priming in a series of two‐session experiments that crossed relatedness proportion (RP, at .25, .50, and .75) with stimulus–onset asynchrony (SOA, at 200, 350, and 800 ms). Low reliability would indicate little coherence of activity within semantic memory, so that the degree to which any given association influences performance is uncorrelated from one association and time of testing to the next. High reliability would indicate that each person tends to harness his or her semantic knowledge consistently, applying semantic relations among words in much the same way from one word to the next and one time of testing to the next to help task performance, resulting in systematic individual differences. What we observed was low reliability—often zero. When conditions highlighted “automatic activation” (low RP, short SOA), priming was completely uncorrelated from item to item and session to session. Both split‐half and test–retest reliability increased to significant levels under conditions that raised the probability that an activated or retrieved episode of prime experience would help with target recognition, and the probability of intentionally generating the target from the prime. Thus, task‐relevant" utility imposes a modicum of order on semantic associations that are otherwise noisy and uncoordinated. Harnessing semantic memory is like herding cats—without considerable constraint, associations tend to come and go their own ways in independent fashion.